This CL replaces most uses of the V8_EMBEDDED_BUILTIN define
by a new read-only runtime flag called FLAG_embedded_builtins.

The flag is true iff V8_EMBEDDED_BUILTINS is defined.

Bug: v8:6666
Change-Id: Ifcc909dc9b028a2c967f8a0e45029df5e71072df
Reviewed-on: https://chromium-review.googlesource.com/1122401
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54156}